I'm giving up after almost one week. I made the changes both manually and using EasyESF. After I change Rome to a 5-slot:
a) I play as the Italian States normally, except any saved games will not load, crashing after staying a bit at 15%;
P.S.: If I play as them without changing normally, it loads fine.
b) I play as any other faction, and nothing happens. Rome's AI works fine, there are no crashes even after it finishes building their admiralty.
I don't know what else to do here.